PROJECT SUMMARY Background and aims: Microvillus Inclusion Disease (MVID) is a form of congenital diarrhea caused by inactivating mutations in Myosin Vb (MYO5B). The available treatment options for individuals with MVID are either lifelong total parenteral nutrition or full intestinal transplantation. MVID patients frequently present with cholestasis. This cholestasis was previously thought to arise from prolonged administration of total parenteral nutrition. However, recent publications have described mutations in MYO5B that do not result in MVID, but instead patients have isolated cholestasis. Given the complex nature of MVID and the limited treatment options available, understanding the pathogenesis of cholestasis resulting from mutations in MYO5B represents an important scientific question which needs to be addressed. The central hypothesis of this research proposal is that mutations in MYO5B result in aberrant expression of apical/canalicular membrane transporters preventing the normal secretion of bile salts. We base our hypothesis on preliminary data generated from our mouse and pig models of MVID which demonstrate mislocalization of BSEP and other canalicular transporters. Our findings are mirrored by published reports from patients with mutations in MYO5B, demonstrating aberrant expression of the bile salt export protein (BSEP) in hepatocytes. To date, no experimental animal model has been used to define the mechanism of cholestasis in the setting of MVID. For this K01 Career Development Award, I propose the use of germline MYO5B KO mice, an MVID pig model and a novel mouse model of MYO5B point mutation to address deficits in apical transporters in hepatocytes that arise from mutations in MYO5B. Specific Aim 1 will define the function of MYO5B in maintaining hepatocyte polarity and homeostasis. Specific Aim 2 will determine the mechanism by which the C266R mutation in MYO5B contributes to cholestasis, but does not result in MVID. At the completion of these studies I expect to have elucidated the role of MYO5B in the regulation of protein trafficking in hepatocytes in vivo and in vitro. This proposal highlights the need for a better understanding of the function of MYO5B in hepatocytes with the ultimate goal of improving current therapeutic treatments for MVID. Long-term objective and aims: Being the recipient of a K01 Career Development Award would provide the mentorship, training and support necessary to achieve my goal of becoming an independent investigator. This research is well suited for the National Institute of Diabetes and Digestive and Kidney Diseases as it relates to digestive and liver disorders. Vanderbilt University Medical Center offers all of the scientific resources required to complete this proposal. I have assembled a group of renowned scientists to serve as my mentors and mentorship committee, and as collaborators. Additionally, I have developed a training plan to enhance my scientific repertoire, increase my publication record and secure independent funding. This will ensure success in securing an independent position to start a new laboratory by the completion of this award.